1. Field of the invention
The present invention relates to a method of manufacturing a double-sided wiring substrate and, more particularly, to a method of efficiently forming fine wiring patterns having a low resistance on both surfaces of a substrate.
2. Description of the Related Art
In a conventional double-sided wiring substrate having surfaces on both of which electric conductive patterns (hereinafter wiring patterns) are respectively formed, a through hole which extends from the upper surface to the lower surface of the substrate is formed by, e.g., punching or drilling, and a metal such as copper is plated on the inner wall of the through hole to connect the wiring patterns formed on both the surfaces. In such a method, however, a metal such as copper cannot be directly plated on the inner wall of the through hole, and hence a pretreatment or undercoating treatment must be performed in advance for the inner wall of the through hole. Therefore, the plating process is complicated, and manufacturing cost is undesirably increased.
Recently, the following method has been considered. That is, after a substrate is pierced with a metal pin to form a pin hole which extends from the upper surface to the lower surface, the pin hole is filled with an electric conductive paste to connect wiring patterns respectively formed on the upper and lower surfaces. The above technique is disclosed in Unexamined Japanese Patent Publication Nos. 60-208888 and 60-208894.
In the technique disclosed in the above applications, a carbon ink is used as an electric conductive paste. Since the carbon ink has a high electric resistivity, it cannot be applied to a low-resistance circuit. In addition, as is conventionally known, when a conductive patterns is formed with the carbon ink, it may often be stained or blurred to disable formation of fine wiring patterns.